The present invention relates to the field of communications in general, and more particularly, to determining the position of a mobile device.
It may be desirable, and in certain places mandated by law, that mobile telecommunication network providers be able to determine an approximate geographical location of a mobile terminal (MT), such as, for example, an actively communicating cellular telephone.
A variety of MT location techniques have been proposed. These location techniques include uplink signal location, downlink signal location, Global Positioning System (GPS) based approaches and approaches based on digital television signals. For “uplink signal” location techniques, the mobile telecommunications network is typically configured to determine where the MT is located based on ranging measurements associated with one or more uplink signals. These uplink signals are transmitted by the MT and received by a requisite number of receivers having known locations, such as, for example, cellular telephone base stations (BSs). For the “downlink signal” location techniques, the mobile telecommunications network is typically configured to determine where the MT is located based on ranging measurements associated with the reception, by the MT, of downlink signals from a requisite number of transmitters having known locations.
The other location approaches generally use location services not associated with either the uplink or downlink signals used in the mobile telecommunications network. In a typical GPS application, the GPS receivers collect and analyze ranging measurements from signals transmitted by GPS satellites having known locations. More specifically, a constellation of 24 satellites orbiting the earth continually emit a GPS radio signal. A GPS receiver, e.g., a hand-held radio receiver with a GPS processor, receives the radio signals from the closest satellites and measures the time that the radio signal takes to travel from the GPS satellites to the GPS receiver antenna. By multiplying the travel time by the speed of light, the GPS receiver can calculate a range for each satellite in view. Ephemeris information provided in the satellite radio signal typically describes the satellite's orbit and velocity, thereby generally enabling the GPS processor to calculate the position of the GPS receiver through a process of triangulation. It is known to include a GPS receiver in a mobile terminal to provide position location functionality to the mobile station.
The startup of a GPS receiver typically requires the acquisition of a set of navigational parameters from the navigational data signals of four or more GPS satellites. This process of initializing a GPS receiver may often take several minutes. The duration of the GPS positioning process is directly dependent upon how much information a GPS receiver has initially. Most GPS receivers are programmed with almanac data, which coarsely describes the expected satellite positions for up to one year ahead. However, if the GPS receiver does not have some knowledge of its own approximate location, then the GPS receiver cannot find or acquire signals from the visible satellites quickly enough, and, therefore, cannot calculate its position quickly. Furthermore, it should be noted that a higher signal strength is typically needed for capturing the C/A Code and the navigation data at start-up than is needed for continued monitoring of an already-acquired signal. It should also be noted that the process of monitoring the GPS signal may be significantly affected by environmental factors. Thus, a GPS signal which may be easily acquired in the open typically becomes harder to acquire when a receiver is under foliage, in a vehicle, or worst of all, in a building.
More recently, it has been proposed that digital television signals could be used for location of a mobile terminal. As described in “Positioning Using the ATSC Digital Television Signal,” Rabinowitz, M. and Spilker, J., Rosum Corporation Whitepaper, www.rosum.com (circa 2001), digital television signals may be broadcast, at least in the United States, from terrestrial digital television transmitters having determinate locations. The Rosum Corporation Whitepaper, proposes a technique for determining range information to digital television transmitters using the synchronization fields of the digital television signal.
These various known location techniques may include collecting ranging measurements such as, for example, a time of arrival (TOA), a time difference of arrival (TDOA), an observed time difference (OTD), or the like. These ranging measurements are typically gathered by detecting one or more measurement features within the transmitted/received signal(s). Each of the various location techniques has certain limitations on their accuracy. By way of example, various TOA, TDOA, and OTD location techniques that utilize existing BSs typically require that at least three (3) or more BSs receive the transmitted uplink signal from the MT, or, conversely, that the MT receive transmitted downlink signals from at least three BSs to perform the locating process. Similarly, with respect to the GPS approach, a GPS receiver generally needs to receive transmitted signals from at least four (4) GPS satellites to perform the complete locating process (although some information may be generated based on transmitted signals received from three GPS satellites).
Moreover, there is not always a clear line-of-sight (LOS) between the MT and the requisite number of known location transmitter(s)/receiver(s). For example, in an urban environment, the LOS is often blocked by building and/or other structures, while in certain other environments the naturally occurring terrain and/or other features (e.g., mountains, canyons, forests, weather, etc.) can reduce the LOS, attenuate the transmitted signals, or produce multipath signals at the receiver. For many higher frequency signals or weaker signals, the loss of LOS or the introduction of such obstacles, can render the location technique significantly inaccurate, or completely unavailable.